Processes of producing aldehydes by hydroformylation of an olefinic unsaturated compound with carbon monoxide and hydrogen in the presence of a Group VIII metal complex catalyst are widely industrialized. As the catalyst in this hydroformylation reaction, complex catalysts comprising a Group VIII metal such as rhodium, modified with a ligand such as compounds of trivalent phosphorus are used, and for enhancing activity and selectivity of the hydroformylation reaction, various ligands are studied. For example, JP-B-45-10730 discloses that rhodium complex catalysts modified with a trivalent phosphorus ligand such as triarylphosphines and triaryl phosphites are effective. Above all, it is known that catalysts modified with a phosphite ligand exhibit high activity and sensitivity in the hydroformylation reaction.
However, as disclosed in JP-A-59-51229, it is known that in phosphite ligands such as triphenyl phosphite, the ligand relatively rapidly decomposes in the hydroformylation reaction system, resulting in a reduction of catalytic activity, and it is necessary to continuously supplement the phosphite ligands. Accordingly, not only for improving the activity and selectivity of the catalyst but also for minimizing the reduction of catalytic activity due to a loss of the phosphite ligands, there are proposed various kinds of phosphite ligands.
For example, cyclic phosphite ligands containing a phosphorus atom in the bridge head portion thereof (JP-A-59-51228 and JP-A-59-51230), triaryl phosphite ligands having a substituent in a specific site of the benzene ring (JP-A-57-123134), triaryl phosphite ligands having a substituent in a specific site of the naphthyl ring (JP-A-4-288033), and diorganophosphite ligands having a cyclic structure containing a phosphorus atom within a molecule thereof (JP-T-61-501268) are proposed. In addition, there are known methods of using, as examples of bisphosphite ligands and polyphosphite ligands, diorganophosphite ligands (JP-A-62-116535 and JP-A-62-116587) and bisphosphite ligands having a cyclic structure (JP-A-4-290551) and a method of using cyclic structure-free bisphosphite ligands and polyphosphite ligands by the present applicant (JP-A-5-178779).
However, as described previously, nevertheless the phosphite ligands exhibit high activity and excellent selectivity in the hydroformylation reaction, for industrially advantageously producing aldehydes, stability of the phosphite ligands themselves was problematic. That is, rapid decomposition of the phosphite ligands involved problems such that not only it adversely affects the activity and stability of catalyst, but also new phosphite ligands must be continuously supplemented.
In addition to the foregoing JP-A-59-51229, for example, JP-T-61-501268 describes that triphenyl phosphite rapidly reacts with an aldehyde at room temperature even in the absence of rhodium. It is thought that a defect of triorganophosphites such as triphenyl phosphite is caused by the matter that the triorganophosphites have a very high affinity to react with aldehydes. Further, it is described that products obtained by reaction of triorganophosphites with aldehydes are readily hydrolyzed to form corresponding hydroxyalkylphosphonic acids. In diorganophosphites, it is described that though the formation speed of products obtained by reaction of diorganophosphites with aldehydes is slow, acid by-products are formed like the foregoing case.
Such hydroxyalkylphosphonic acids are formed by an autocatalytic process, and especially, are liable to be formed in a continuous catalytic recirculation process wherein contact of phosphite ligands with aldehyde products extends over a long period of time. Since such hydroxyalkylphosphonic acids are in general insoluble in liquid hydroformylation reaction media, they are rapidly accumulated in the process to precipitate gelatin-like by-products, so that they may possibly clog or stain circulation conduits of the continuous hydroformylation reaction.
For removing such precipitates by an arbitrary proper method such as a method of acid extraction with weak bases such as sodium bicarbonate, it is necessary to periodically stop or pause the operation of the process. It may be said that such a phenomenon is a characteristic feature inherent to phosphite based ligands, which is not seen in conventionally industrially employed phosphine based ligands such as triphenylphosphine.
As methods of solving the problem of stability of these phosphite ligands, for example, JP-A-60-156636 discloses a method of adding tertiary amines for neutralizing acidic substances formed by decomposition of phosphite ligands. Further, JP-T-61-501268 discloses a method of minimizing decomposition of phosphite ligands by removing acidic substances with weakly basic anion exchange resins. In addition, JP-B-5-48215 discloses that metallization of rhodium is depressed by distillation in the presence of a specific polar functional group-containing organic polymer and discloses that in distillation and separation of aldehyde products from reaction products containing a rhodium-phosphite based complex catalyst, it is desired that the distillation and separation are carried out at a temperature of lower than 150° C., and preferably lower than 140° C. JP-A-6-199729 discloses a method of stabilizing phosphite ligands against decomposition by adding epoxides. JP-A-6-199728 discloses a method of using added water and/or weakly acidic additives as additives for enhancing catalytic activity of a specific rhodium-bisphosphite complex catalyst. Moreover, JP-A-8-165266 discloses a method in which in separating at least one component selected from carbon monoxide, hydrogen, an unreacted olefinic unsaturated compound, an aldehyde product, a solvent, a middle boiling point by-product, and a high boiling point by-product from a reaction product containing the aldehyde product by a separation operation, the separation operation is carried out within a certain defined range of a parameter obtained from the temperature and residence time in the separation operation, whereby a loss of phosphite ligands and formation of by-products are effectively suppressed. Also, it is disclosed that in the case where the foregoing separation operation is steam distillation, when the separation operation is carried out within a certain defined range of a parameter obtained from the steam distillation temperature, residence time and steam fraction in the separation operation, decomposition of the phosphite ligands is suppressed.
In the light of the above, in the conventional technologies, some added substances or post-treatment methods were required. Further, though ones in which the operation conditions in the separation step are defined were known, they did not provide a process of substantially suppressing the decomposition of phosphite ligands.
Further, the foregoing JP-A-6-199728 discloses that a part of bisphosphite ligand catalysts is accelerated in catalytic activity by the addition of water. But, in general, it is known that as in triphenyl phosphite described in JP-T-61-501268, decomposition products of phosphite ligands are formed in the presence of water, and further decomposition of the decomposition products proceeds.
However, a little of water is in general present in the hydroformylation reaction system. This is because not only in the hydroformylation reaction system, a condensation dehydration reaction takes place as a side reaction to form water as a by-product, but also water that is entrained with a mixed gas of hydrogen and carbon monoxide (the mixed gas being hereinafter referred to as “oxo gas”) as raw materials and incorporated in the hydroformylation reaction system is not negligible. The concentration of water to be entrained in the oxo gas varies depending on the kind and operation conditions of the oxo gas manufacture step. For example, in the case where methane or naphtha is subjected to a steam reforming reaction and a water gas reaction, or a partial oxidation reaction together with carbon dioxide, water vapor, etc. at high temperatures of about 800° C. to obtain a decomposed gas comprising hydrogen, carbon monoxide, carbon dioxide, water vapor, etc., and the decomposed gas is then introduced into an absorption column and subjected to absorption and removal of carbon dioxide by an alkanolamine or a hot potassium carbonate aqueous solution (hereinafter referred to as “decarbonation step”) to obtain a purified oxo gas, since the obtained purified oxo gas contains a saturated water vapor under the operation pressure and temperature conditions of the absorption column in the decarbonation step, even when a major part of water is removed by compression and cooling condensation in an after step, from 0.2 to 0.7% by volume of water is generally carried as the water vapor, and therefore, this water is incorporated into the hydroformylation reaction system. Further, in the step of separating and recovering the catalyst, when a catalyst-containing solution (hereinafter referred to as “catalyst liquid”) is subjected to contact processing with water such as water washing and then circulated and used again for the hydroformylation reaction, water of about a saturated solubility is at least contained in the catalyst liquid, and therefore, when the catalyst liquid is directly fed into the hydroformylation process, water is carried into the process.
In the light of the above, in the hydroformylation reaction using a rhodium-phosphite based complex catalyst, decomposition of phosphite ligands proceeds by water in the process, causing a reduction of the activity of the catalyst.
In addition, it is known that in the hydroformylation reaction using a phosphite based complex catalyst, the reaction temperature can be made low as compared with the case of using a phosphine based complex catalyst because the phosphite based complex catalyst has an activity higher than the phosphine based complex catalyst. Thus, the temperature in the process becomes low, and the amount of water vapor to be purged outside the process becomes small, and therefore, it is thought that the amount of water in the process is high as compared with the case of using the phosphine based catalyst.
As the hydroformylation process, there are generally known a liquid circulation type hydroformylation process in which an olefinic unsaturated compound is continuously reacted with carbon monoxide and hydrogen in the presence of a catalyst, and a reaction product containing the catalyst and an aldehyde product taken out from a reactor is fed into a catalyst separation step to separate the aldehyde product, followed by again circulation into the reactor; and a fixed catalyst type hydroformylation process in which an olefinic unsaturated compound is continuously reacted with carbon monoxide and hydrogen in the presence of a catalyst, a reaction product containing the aldehyde product, unreacted olefinic unsaturated compound and by-products taken out from a reactor is fed into a separator to separate the aldehyde product, and the residue is recirculated into the reactor.
As a method of reducing the amount of water in the process, there may be considered a method in which after separating the aldehyde product from the reaction product containing the catalyst and aldehyde product taken out from the reactor, a part of water is taken outside the process together with the catalyst contained in the reaction product, thereby reducing the amount of the catalyst liquid to be circulated into the reactor, or a method of newly providing a dehydration device. However, according to these methods, a loss of the catalyst becomes large, or the cost for equipment increases, and therefore, these methods are not economical.
Under these circumstances, the invention has been made, and its object is to provide a process of suppressing decomposition of phosphite ligands within a catalyst-existent region in the continuous hydroformylation process using a general rhodium-phosphite based complex as a catalyst, especially to provide a process of producing aldehydes efficiently and economically by reducing water within a catalyst-existent region that will become a cause of decomposition of phosphite ligands.
The present inventors made extensive and intensive investigations about the foregoing problem. As a result, it has been found that in the continuous hydroformylation process, when at least a part of an aldehyde product and water are taken out as a mixed vapor flow from a catalyst-existent region of the hydroformylation process, and at least a part thereof is fed outside the catalyst-existent region and treated as it stands as the vapor or as a condensate after cooling, it is possible to efficiently and economically reduce the water concentration within the catalyst-existent region in the process without causing an increase of a loss of the catalyst or without a need of newly providing a dehydration device, resulting in attaining the invention.